Humans see through the cornea, the clear central part of the front surface of the eye. Normally the cornea is dome shaped. Sometimes, however, the structure of the cornea is not strong enough to hold the dome shape and bulges outward in a cone shape. When this occurs, the condition is called keratoconus. Keratoconus is a progressive disease characterized by a thinning of the cornea with concomitant change in the shape of the corneal surface with resultant manifestation of irregular astigmatism and reduced best spectacle corrected visual acuity. The progressive thinning frequently results in severe ectasia, corneal scarring, and visual compromise that requires penetrating keratoplasty to restore vision.
The cause of keratoconus is unknown. It may run in families and does occur more often in people with certain medical problems, including certain allergic conditions. Keratoconus usually begins in the teenage years, but can begin as late as age thirty. Changes in the shape of the cornea occur gradually, often over several years. Over time, the patient's vision slowly becomes distorted. Both eyes are eventually affected, even though at first only one eye may be affected. The extent of change may vary between the eyes. Diagnosis is made using corneal measurements.
A new spectacle can make vision clear in mild cases of keratoconus. Eventually it will probably be necessary to use contact lenses to make vision clear. Typically, rigid contact lenses are used. Contact lenses manufactured for keratoconus have used posterior designs that are geometrically centered in the lens. In the majority of keratoconus cases the apex of the ectatic cornea is not centered, rather, it is displaced, or off-center. Because of this, lenses designed to conform to the shape of the cornea displace in the direction of the apex displacement as the lenses attempt to fit the underlying cornea. Rigid contact lenses are frequently used to provide a uniform optical surface and normal best corrected visual acuity. At the same time, lens intolerance or scarring may occur with these lenses. Hybrid (rigid center-hydrogel skirt) lenses have been suggested as an ideal means of providing the benefits of rigid lenses for correction of the irregular astigmatism by way of the tear lens while providing the comfort and stability of soft lenses.
Recent animal research with lenses designed for purposeful corneal reshaping and corneal refractive therapy demonstrate the potential to increase the thickness of the epithelium with lenses that vault a portion of the cornea. These studies explain the clinical phenomena of correcting hyperopia by applying an lens having a radius of curvature greater than that of the underlying cornea.
There is no evidence in the literature that any contact lens design strategy has been useful in correcting keratoconus by therapeutically increasing the thickness of the epithelium or the underlying cornea by vaulting the cornea. Such a strategy would be difficult with conventional rigid gas permeable or hydrogel contact lenses. The preferred fitting method for rigid gas permeable lenses has been a three point of feather touch of the lens where the lens makes direct contact with the apex or thinnest portion of the cornea in an eye with keratoconus. In hydrogel lenses, the lens often drapes the cornea and makes contact with the majority of its surface.
Attempts have been made to use corneal topography to create a lens that matches the shape of the patient's cornea. In cases or moderate to severe ectasia, these measurements are not reliable and lenses made using these measurements have been known to cause discomfort. The discomfort in turn causes the patient to discontinue wearing the lenses. In addition, corneal topography is approximately limited to the central nine millimeters of the cornea. A well-fitted contact lens for keratoconus must cover a larger area, preferably an area that extends beyond the diameter of the cornea.
Soper, McGuire and Rose K lenses have been commonly used in treating keratoconus patients. These lenses are rigid corneal lenses that are small in diameter, rotationally symmetrical, and use multiple concentric spherical curves. The lenses displace in the direction of the apex of the underlying cornea and provide limited visual acuity due to the residual low order astigmatism and higher order aberrations. Furthermore, the lenses are time consuming to fit and provide limited comfort, wearing time, and vision.
Another design used in keratoconus patients is the Quadra design. This design is a rigid corneal lens with a central apex having different eccentricities in the four quadrants of the posterior surface of the lens. The different eccentricities in the four quadrants accommodate the difference in sagittal height of the ectatic cornea. The Quadra design fits the cornea more closely than lenses using concentric spherical curves. Unfortunately, the Quadra lens also displaces toward the apex of the cornea and provides limited visual acuity and comfort.
Softperm and SynergEyes KC are hybrid contact lenses having spherical and elliptical concentric designs, respectively. The hybrid lenses have a rigid gas permeable center and a soft hydrogel skirt. They are fitted with a central or apical radius shorter than the underlying eye and are designed to vault the apex of the cornea. The hydrogel skirt provides better comfort than purely rigid contact lenses, but still causes discomfort and may exhibit flexure with resulting residual low and higher order aberrations on eyes with moderate to severe ectasia.
The Boston Scleral Lens is a rigid gas permeable lens that is custom fitted. The lens has a central portion that vaults the ectatic cornea. The mid-peripheral and peripheral portion of the lens align the corneal-scleral junction and sclera. This lens design does not displace toward the corneal apex. Fewer residual low and higher order aberrations are found in this scleral lens-eye system, but fitting is quite complex and expensive, leading to limited use of the design. A further drawback is that the lenses are difficult to apply and remove and have been found to be less comfortable than hybrid and soft contact lenses.
The Roffman lens is a hybrid multifocal contact lens incorporating both soft lens material and a rigid lens material. The rigid lens material is wholly contained within the soft lens material. At least one portion of the lens, preferably the front surface, has a multifocal optical zone. The multifocal zone may be a progressive power zone or may contain three to five concentric zones. A drawback to the Roffman lens is that is does not treat keratoconus and as a result would be likely to displace in the direction of the apex of the underlying cornea.
The conventional fitting method for rigid gas permeable lenses is a three point of feather touch of the lens where the lens makes direct contact with the apex or thinnest portion of the cornea in an eye with keratoconus. In hydrogel lenses the lens often drapes the cornea and makes contact with the majority of its surface. In many cases, these lenses are not comfortable for the patient, with the result that the lenses are not worn.